A quinoline compound is very important structural unit in organic chemistry and has important application value in the fields of medicines, pesticides, dyes, functional materials and spices. Since the quinoline has been separated from coal tar by Runge in 1834, how to synthesize quinoline derivatives chemically has been a hotspot for organic synthetic chemists.
Classical chemical methods of organic synthesis of the quinoline derivatives include Skraup-Doebner-von Miller synthesis, Combes-Conrad-Limpach synthesis, Friedländer synthesis and Pfitzinger synthesis [Fallah-Mehrjardi M., Mini-Rev. Org. Chem., 2017, 14, 187]. These synthetic methods have a critical disadvantage that reaction is usually conducted in high temperature and strong acid systems, which lead to high requirements for equipment and serious environmental pollution in industrial production. In recent years, a transition metal-catalyzed coupling reaction has been gradually concerned for its high efficiency, and has become an important method for synthesis of the quinoline compound [Prajapati S. M., Patel K. D., Vekariya R. H., Panchal S. N., Patel H. D., RSC Adv., 2014, 4, 24463]. Although the transition metal-catalyzed method avoids the use of inorganic acid such as concentrated sulfuric acid or concentrated hydrochloric acid, a transition metal catalyst still has the defect of the difficulty in separation and recovery of the catalyst in the process of catalytic synthesis of the quinoline derivatives. Therefore, it is of great significance to develop a green synthetic method of quinoline compounds. A metal-free catalytic reaction as an important green synthetic method has attracted much attention. A method for preparing quinoline from N-benzyl aniline by using radical cation salt as a catalyst was reported in 2015, but the development of the method was limited due to difficulty in synthesis of the catalyst [Liu J., Liu F., Zhu Y., Ma X., Jia X., Org. Lett., 2015, 17, 1409].